During the 1983 high water year, the spillway tunnels at Glen Canyon Dam were used to convey a large volume of water downstream to prevent the rapidly rising reservoir from overtopping the dam. Because the original design of the spillway tunnels did not include aeration slots, cavitational forces from the water, eroded the concrete inside the tunnels and caused significant damage.

Cavitation is the formation of partial vacuums or cavities in fast-flowing water which wears away solid surfaces such as concrete, as a result of the collapse of these vacuums. The subsequent repair of the Glen Canyon Dam spillway tunnels included aeration slots which inject air at the base of the water flow to prevent cavitation from occurring.

As a result of the lesson learned at Glen Canyon Dam, Reclamation retrofit other dam spillway tunnels with similar aeration slot designs. The spillway at Flaming Gorge Dam on the Green River in northeastern Utah was modified in the mid-1980’s not long after the Glen Canyon spillway. Although the Flaming Gorge Dam spillway has rarely been used, a periodic inspection of the spillway revealed damage to the concrete in the area where tunnel was modified to install the aeration slot, about four feet above the edge of the aeration slot itself.

Reclamation inspectors believe the damage is most likely due to effects of the hot and cold temperature fluctuations on the concrete. The damage was severe enough that a recommendation was made to repair the damaged concrete. Members of Reclamation’s Upper Colorado Region Power Office rope access team from both Flaming Gorge Dam and Glen Canyon Dam, along with other support staff, worked together to conduct the repairs.

In 2008, these teams were also involved with making similar concrete repairs to the spillway tunnel at Yellowtail Dam in Montana where similar temperature conditions to those at Flaming Gorge, damaged the concrete in the area of the aeration slot. With funding from Reclamation’s Montana Area Office, Reclamation’s Technical Services Center concrete lab identified a quick-drying, pre-combined, polymer mortar that was first used to repair the Yellowtail Dam spillway as well as the Flaming Gorge Dam spillway.

The rope access team reached the damaged concrete in the Flaming Gorge spillway by rappelling approximately 187 feet down the spillway tunnel which slopes at a 55 degree angle and is 20 feet in diameter at the point of the repair. Crews used an air compressor and rubber material to divert the small amount of water that flows down the spillway, around the repair area. The process took approximately two weeks to complete and the repaired area is expected to remain in good condition for a long period of time.